The present inventions relates to a process for selectively compressing granular material in a molding box and, more particularly, a process wherein the granular material is compressed by a precompression step followed by a final compression, the precompression step being carried in two stages.
The packing granular molding materials for the purposes of producing factory molds is old and well-known in the prior art. It is customary when packing the granular molding materials to provide a pre-determined amount of a compressed medium over the molding box which acts upon the granular material in the molding box to compress same. A typical process for packing granular molding material is to disclosed in German Patent No. 19 61 234 wherein the packing of the molding material is effected by the expansion of a high pressure gas within the molding apparatus. During the packing process, the gas, which is stored under pressure in a reservoir, is transferred via a valve within a very short time interval to the surface of the granular material so as to impact on the surface of the material being compressed. The pressure is maintained on the surface of the granular material for a predetermined time interval and thereafter the pressure is reduced by means of venting.
U.S. Pat. No. 4,828,007 discloses an improved process for selectively compressing granular material in a molding box. The process of the 4,828,007 patent relies on two sequential pressure surges for final compaction of the molding sand wherein the pressure gradient of the first pressure surge is smaller than the pressure gradient of the second pressure surge.
It has been found by the inventors that, when dealing with mold patterns having significant height differences in complex pattern contour, prior art processes do not always yield homogeneous compacted molds having high hardness values.
Naturally, it would be highly desirable to provide a process which allows for uniform compression of sand molds of high hardness when employing mold patterns having complex pattern contour and significant height differences.
Accordingly, it is a principal object of the present invention to provide a process for compressing granular materials in a molding box.
It is a particular object of the present invention to provide a process as aforesaid employing a precompression step followed by a final compression step. It is a further particular object of the present invention to provide a process as set forth above wherein the precompression step is carried out by the sequential application of a first pressure surge followed uninterruptedly by a second pressure surge wherein the rate of pressure change over time of the second pressure surge is greater than that of the first pressure surge.
Further objects and advantages of the present invention will appear hereinbelow.